Brake system with hydraulic brake booster and corresponding method

ABSTRACT

A brake system with hydraulic brake boosting has a hydraulic actuating unit which comprises a hydraulic reservoir, a brake pedal, a brake-pressure generator and a pedal-travel simulator, a hydraulic unit comprising a pressure generator, a hydraulic wheel-brake device and a control unit for coordinating the functions of the actuating unit, of the hydraulic unit and of the wheel-brake device. The pedal-travel simulator possesses a variable displacement volume which is connected to the hydraulic reservoir via a hydraulic line. 
     In order to design electrohydraulic brakes by simple means so as to be more fail-safe, a return line is provided between the wheel-brake device and the displacement volume of the pedal-travel simulator. Moreover, there is arranged in the hydraulic line, between the hydraulic reservoir and the displacement volume of the pedal-travel simulator, a return-flow valve which in the event of a fault, with the brake system defective, can be shifted into a shut-off position blocking the hydraulic line.

This application claims the priority of PCT International No.PCT/EP099/09704 filed Dec. 9, 1999 and German Patent Document 199 02444.8, filed Jan. 22, 1999, the disclosures of which are expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a brake system with hydraulic brakeboosting and to a process therefor.

DE 35 26 556 A1 discloses such a brake system. The master brake cylinderis connected to the wheel brake device by means of a brake line by wayof a multi-position valve. A servo brake pressure supplying device canbe connected by way of the multi-position valve and can feed additionalbrake pressure to the wheel brake device. A branch line branches offfrom the brake line between the master brake cylinder and themulti-position valve. Said branch line is connected to the pedal travelsimulator and then shut off by means of a solenoid valve, when a faultoccurs in the brake system, for example in the servo brake pressuresupplying device.

The pedal travel simulator is supposed to achieve a meterable brakepedal behavior that is familiar to the driver and to reach a pedaltravel distance that is significant during braking.

When a fault occurs, the pedal travel simulator is completely separatedwith respect to the hydraulic medium. In this respect the hydraulicmedium, located in the pedal travel simulator at the time of theseparation by means of the solenoid valve, is locked in and is notavailable for further brake pressure build-up, for example, in thesubsequent braking operations.

The DE 35 26 556 A1 discloses a brake system for motor vehicles, whichcomprises a master cylinder or a brake pressure generator, which isdesigned as a tandem cylinder and which is actuated by a brake pedal anddrives a wheel brake device with pressurized hydraulic medium, deliveredto the wheel brake device from a reservoir. Provided is also anauxiliary pressure supply system that comprises a hydraulic pump and anelectric motor, actuating the pump. The medium, conveyed by the pump,can be fed to the wheel brake device via a switching valve.

A pressure chamber of the master cylinder, acted upon by the brakepedal, communicates with the pedal travel simulator, which is designedas a cylinder with a displaceable actuating piston. The actuating pistonlimits, in the cylinder, a displacement volume, which is connected tothe reservoir via a hydraulic line.

When the volume of the pressure chamber of the master cylinder isreduced due to the pressure of the brake pedal, the medium flows out ofthe pressure chamber into the pedal travel simulator and acts on theactuating piston, which is adjusted in the direction of the displacementvolume. In so doing, the medium, contained in the displacement volume,escapes into the reservoir. At the same time the pressure in thedisplacement volume of the pedal travel simulator is used for thedynamic modulation of the brake pressure, which is generated via themotor-operated hydraulic pump of the auxiliary pressure supply system.

In order to provide sufficient brake force in the event of a failure ofthe power supply, the brake pressure in this kind of system is notgenerated, in the event of a fault, by means of the auxiliary supplysystem, but rather directly by means of the master cylinder. It ishereby possible to prevent hazardous situations, which may arise due toa failure of the hydraulic pump, for example as a result of a breakdownin the power supply or a failure of the control unit of the system.

If in brake systems, described above, a fault occurs during a brakingoperation, in which the brake pedal is already partially depressed,there is the problem that up until the brake pedal stops only theremaining residual pedal travel is available via the master cylinder forthe build-up of brake pressure necessary during emergency situations.This pedal travel is reduced, as compared to the non-actuated brakepedal, and, under some circumstances, is not sufficient for generatingthe necessary brake force. If the brake pedal is already fully depressedat the moment when the fault occurs, no additional brake pressure can begenerated at all by way of the master cylinder.

SUMMARY OF THE INVENTION

An object of the present invention is to provide electrohydraulic brakeswith simple construction so as to be more fail safe.

This object has been achieved according to the present invention by thebrake system and process described herein.

More specifically, a return line is arranged in the hydraulic pathbetween the wheel brake device and the displacement volume of the pedaltravel simulator. After the end of the braking operation, said returnline conveys the hydraulic medium out of the wheel brake device normallyfirst into the displacement volume and then by means of anotherhydraulic line back into the reservoir. The hydraulic line between thedisplacement volume and the reservoir has a return flow valve, which canbe adjusted between an opening position, releasing the hydraulic line,and a shut-off position, closing the hydraulic line. Normally the returnflow valve is in the open position so that the hydraulic medium flowsout of the wheel brake device into the reservoir.

In contrast, in the event of a fault, the return flow valve is shiftedinto the shut-off position so that, although the connection between thewheel brake device and the displacement volume of the pedal travelsimulator is still open, the connection between the displacement volumeand the reservoir is interrupted and is consequently hydraulicallyrigid. The result is that in the event of a fault occurring during abraking operation, the energy stored in the wheel brake device causesthe hydraulic medium to flow out of the wheel brake device through thereturn line back into the displacement volume of the pedal travelsimulator. Said displacement volume fills with hydraulic medium andincreases its volume as a result of the interrupted connection to thereservoir. The result of the increase in volume of the displacementvolume is that the brake pedal, connected to the displacement volume, isset in the direction of its initial position so that the brake pedaltravel available to the driver is increased automatically. Theadditional pedal travel can then be utilized for the brake force, whichis generated by the driver and which in the event of faults passes ashydraulic brake pressure through to the wheel brake device directly viathe actuating and hydraulic unit.

In an advantageous development, the return-flow valve in the linebetween the displacement volume and the reservoir is capable of beingactuated electrically and, in the dead state, to be in a shut-offposition blocking the throughflow. This ensures that, in the event of afault, the displacement volume is always cut off from the reservoir andtherefore the medium conveyed back into the displacement volume mustcollect in the displacement volume and necessarily leads to an expansionof this volume.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,wherein the sole FIGURE is a schematic diagram of the brake systemconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The brake system 1 is an electrohydraulic brake and includes anactuating unit 2, a hydraulic unit 7 connected to the actuating unit 2and a wheel-brake device 8 for the four vehicle wheels 14, 15, 16, 17.The wheel-brake device 8 is supplied with hydraulic medium from thehydraulic unit 7 and has four brake units which are assigned in eachcase to a vehicle wheel 14 to 17.

The actuating unit 2 includes, in particular, a hydraulic reservoir 3, abrake pedal 4, a brake-pressure transmitter 5 and a pedal-travelsimulator 6. The hydraulic reservoir 3 supplies the pedal-travelsimulator 6 and the hydraulic unit 7 with hydraulic medium via hydrauliclines 11 and 18. The pedal travel of the brake pedal 4 is recorded by atravel sensor 19; alternatively to a travel sensor, a force sensor mayalso be provided to measures the pedal pressure exerted on the brakepedal 4 by the driver.

The brake-pressure generator 5 acted upon by the brake pedal 4 isconventionally configured as a hydraulic cylinder, for example as atandem cylinder, and is connected to the hydraulic unit 7 via a line 20.In the event of faults, with the brake system defective, the pedalpressure of the brake pedal 4 is converted directly into hydraulic brakepressure via the line 20 and is fed to the wheel-brake system 8 via thehydraulic unit 7.

The hydraulic unit 7 includes, in order to generate the brake pressureto the necessary level, a conventional pressure generator, notillustrated, usually an electrically driven hydraulic pump. Thehydraulic pressure generated in the hydraulic unit 7 is fed as brakepressure to the brake units of the wheel-brake device 8 for the vehiclewheels 14 to 17 via lines 26 to 29.

The brake-pressure generator 5 is connected to a pedal-travel simulator6 via a line 21. The pedal-travel simulator 6 performs the function ofsimulating an elastic, resilient and accurately meterable behavior ofthe brake pedal 4. The pedal-travel simulator 6 has a hydraulic cylinderwith an adjusting piston 22 and of a displacement volume 10 which can beset variably via the adjusting piston 22 as a function of the pedalposition of the brake pedal 4. The adjusting piston 22 is loaded by aspring 30 with spring force which urges the adjusting piston 22 into aposition which increases the displacement volume 10 and in which thebrake pedal 4 assumes its initial position. The displacement volume 10is connected to the hydraulic unit 7 via a return line 12 and to thereservoir 3 via the hydraulic line 11. An electrically actuablereturn-flow valve 13 is arranged in the hydraulic line 11. In the soleFIGURE, the electrically actuable return-flow valve 13 is shown in theclosed position.

A control unit 9 is provided to control and coordinate the functioningof the individual components of the brake system. In particular, via acontrol line 23, the unit 9 records measurement signals from the sensor19 or receives them as input signals and, via a control line 24,transmits output signals to the electric motor in the hydraulic unit 7or controls the electrical return-flow valve 13 via a control line 25.

In the normal braking mode, with the brake system intact, depending onthe design of the sensor 19, the pedal travel or the pedal pressure ofthe brake pedal 4 is sensed, and the pedal travel or the pedal pressureis fed as an input signal to the control unit 9. A corresponding brakepressure is determined from the pedal travel or from the pedal pressurein the control unit 9 and an output signal corresponding to the brakepressure is fed to the hydraulic unit 7 in order to set the pump motorof the hydraulic pump. The hydraulic medium required is fed to thehydraulic unit 7 from the reservoir 3 via the line 18.

Furthermore, during or after each braking operation, the electricallyactuable return-flow valve 13 in the hydraulic line 11 between thepedal-travel simulator 6 and the reservoir 3 is opened. Thereby, afterthe end of the braking operation, the hydraulic medium stored in thebrake units of the wheel-brake device 8 can be led back into thereservoir 3 via the return line 12, the displacement volume 10 of thepedal-travel simulator 6 and the hydraulic line 11. At the same time,the adjusting piston 22 of the simulator 6 is shifted into its initialposition by the spring 30, with the result that the brake pedal 4resumes its initial position again.

The electrically actuable return-flow valve 13 is configured so that, inthe dead or non-controlled state, the valve is in the shut-off position,in which the hydraulic line 11 leading to the reservoir 3 is blocked. Inorder to open the hydraulic line 11, the return-flow valve 13 must befully operational and must be acted upon by an actuating signal from thecontrol unit 9. If the correct actuating signal for the return-flowvalves 13 is not generated in the control unit 9 or if the power supplyfails, the return-flow valve 13 cannot be shifted into its openingposition allowing the through flow through the hydraulic line 11; thereturn-flow valve 13 remains in its blocking shut-off position, in whichthe connection between the pedal-travel simulator 6 and the reservoir ishydraulically rigid.

If a fault occurs in which the control unit 9 fails and/or the powersupply to the assemblies to be actuated electrically fails, thereturn-flow valve 13 cannot be adjusted into the opening position. Theresult of this, after the end of the braking operation, is that theconnection between the displacement volume 10 and the reservoir 3remains interrupted. The hydraulic medium conveyed to the pedal-travelsimulator via the return line 12 as a result of the higher brakepressure of the brake units of the wheel-brake device 8 flows into thedisplacement volume 10 and, even when the brake pedal is actuated by thedriver, causes the brake pedal to be reset into the initial position. Ifthe power or the control unit fails during a braking operation, it isthereby possible to ensure that, by virtue of the resetting of the brakepedal, the entire pedal travel is available again for the emergencybraking via the brake-pressure generator 5, the line 20 of the hydraulicunit 7.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. Brake system with hydraulic brake boostingcomprising a hydraulic actuating unit having a hydraulic reservoir, abrake pedal, a brake pressure generator and a pedal travel simulatorwith a variable displacement volume, a hydraulic unit comprising apressure generator; a hydraulic wheel brake device; and a control unitfor operatively coordinating functions of the actuating unit of thehydraulic unit and of the wheel brake device, wherein the wheel brakedevice is configured for recirculation of the hydraulic medium and isconnected to the displacement volume of the pedal travel simulator by areturn line, and the displacement volume of the pedal travel simulatoris connected to the hydraulic reservoir, by a hydraulic line torecirculate the hydraulic medium out of the wheel brake device back intothe hydraulic reservoir by way of the displacement volume of the pedaltravel simulator, and the hydraulic line between the hydraulic reservoirand the displacement volume of the pedal travel simulator includes areturn flow valve configured, in the event of a fault from a defectivebrake system, to be shifted into a shut-off position blocking thehydraulic line.
 2. Brake system according to claim 1, wherein the returnflow valve is configured to be electrically actuateable, whereby, in adead state, the return flow valve is in the shut-off position.
 3. Brakesystem as claimed in claim 1, wherein the pedal travel simulator is ahydraulic cylinder with a displaceable adjusting piston for variablesetting of the displacement volume.
 4. Brake system as claimed in claim3, wherein the return flow valve is configured to be electricallyactuateable, whereby, in a dead state, the return flow valve is in theshut-off position.
 5. Brake system as claimed in claim 1, wherein thehydraulic unit is arranged in a flow path between the actuating unit andthe wheel brake device.
 6. Brake system as claimed in claim 5, whereinthe return flow valve is configured to be electrically actuateable,whereby, in a dead state, the return flow valve is in the shut-offposition.
 7. Brake system as claimed in claim 6, wherein the pedaltravel simulator is a hydraulic cylinder with a displaceable adjustingpiston for variable setting of the displacement volume.
 8. Brake systemas claimed in claim 5, wherein the return line is arranged between thehydraulic unit and the displacement volume of the pedal travelsimulator.
 9. Process for boosting brake force in a brake system asclaimed in claim 8, comprising converting pedal travel of a brake pedalinto a brake pressure that acts upon a wheel brake device, using thepedal travel to influence a displacement volume of a pedal travelsimulator; normally, after the end of a braking operation conveying thehydraulic medium out of the wheel brake device by way of a return lineinto the displacement volume of the pedal travel simulator and out ofthe displacement volume of the pedal travel simulator via a hydraulicline into a hydraulic reservoir; and, in the event of a fault during abraking operation, interrupting the hydraulic line between thedisplacement volume and the hydraulic reservoir and collecting thehydraulic medium of the wheel brake device in the displacement volume ofthe pedal travel simulator.
 10. Process as claimed in claim 9, furthercomprising maintaining an electrically actuable return flow valve in thehydraulic line between the pedal travel simulator and the hydraulicreservoir in a dead state in a shut-off position to block the flowthrough the hydraulic line.